This invention relates to the polymerization of unsaturated hydrocarbons over Ziegler-type catalysts, and more particularly, to processes for formulating such catalysts by sequentially mixing the various components thereof and controlling the orders of addition and the durations of mixing such catalyst components.
The polymerization of unsaturated hydrocarbons over Ziegler-type catalysts is well known in the art. Such hydrocarbons normally take the form of short chain alpha olefins such as ethylene, propylene and butylene, including substituted alpha olefins such as substituted vinyl compounds, for example, vinyl chloride or vinyl toluene. However, such unsaturated hydrocarbons can also include di-olefins such as 1-3-butadiene or 1-4-hexadiene or acetylenically unsaturated compounds such as methylacetylene or 2-butyne.
Ziegler-type catalysts incorporate a transition metal, usually titanium, zirconium or hafnium, although other transition metals found in Groups 4, 5 and 6 of the Periodic Table of Elements may be employed, which function to provide sites for the insertion of monomer units into growing polymer chains. One type of such polymerization catalysts are the so-called homogeneous catalyst systems in which the transition metal compound is a metallocene comprising one or more substituted or unsubstituted cyclopentadienyl groups coordinated with the transition metal atom forming the situs for polymer growth. Such metallocene-based catalyst systems are the subject of European Patent Application EP 129,368 and U.S. Pat. Nos. 4,794,096 to Ewen and 4,892,851 to Ewen et al., the latter two patents disclosing catalysts useful in the polymerization of propylene to form isotactic or syndiotactic polypropylene.
The more widely used transition metal catalysts are the so-called heterogeneous catalyst systems in which a transition metal halide, usually zirconium, hafnium or titanium, di-, tri-, or tetra-halides, are incorporated with a support structure, principally based upon magnesium or zinc halides, ethoxides or the like. For example, U.S. Pat. No. 4,476,289 to Mayr et al. discloses so-called xe2x80x9cactivatedxe2x80x9d titanium tetrahalides, more specifically, titanium tetrachloride, supported on anhydrous magnesium or zinc halides, principally magnesium chloride or magnesium bromide. The transition metal component is used in conjunction with a second component, commonly referred to as a co-catalyst, which as described in the Mayr et al. patent, is a hydride or organometallic compound based primarily upon aluminum, although lithium or magnesium based compounds are also disclosed. A supported catalyst containing yet another component is disclosed in U.S. Pat. No. 4,636,486 to Mayr et al. Here, the titanium compound, which may be a halide, an oxyhalide or an alcoholate in either the di-, tri- or tetra-valent form, is composited with the magnesium support, together with an electron donor compound. Such electron donors, commonly referred to as internal electron donors because they are incorporated as part of the transition metal catalyst component, can be selected from a broad class of compounds including amines, amides, phosphines, ethers, thioethers, alcohol esters, aldehydes, and ketones. As in the case of the aforementioned U.S. Pat. No. 4,476,289 to Mayr, the catalyst system here also includes an organoaluminum co-catalyst such as triethylaluminum, commonly referred to as TEAL. Both of the Mayr et al. patents teach that the molar ratio of the organoaluminum compound and the titanium catalyst component is not critical. In the polymerization of ethylene, such ratio is said to preferably be between 50 and 1,000.
Yet a third component often employed in Ziegler-type catalyst systems is a so-called external electron donor. The external electron donors function similarly as the internal electron donors and in a complimentary or supplementary manner to regulate monomer insertion into the polymer chain growing on the transition metal active sites. Thus, the electron donors can have an impact upon catalyst activity, polymer molecular weight, and polymer morphology as reflected in stereospecificity and physical parameters such as melting point. For example, in the polymerization of propylene, the addition of electron donors under controlled conditions can result in dramatic increases in activity (the amount of polymer produced per unit of catalyst) and in stereoregularity, e.g., an increase in isotactic polymer with a corresponding decrease in atactic. The most widely used external electron donors are organosilicon compounds such as organosilanes and organosiloxanes, including silyl ethers and esters such as alkyl or arylalkyl alkoxysilanes.
The complimentary nature of the internal and external electron donors is addressed in Soga, K. et al., xe2x80x9cEffect of Diesters and Organosilicon Compounds on the Stability and Stereospecificity of Ziegler-Natta Catalystsxe2x80x9d, Transition Metal Catalyzed Polymerizations: Ziegler-Natta and Metathesis Polymerizations, Quirk, R. P., Ed., Cambridge University Press, New York, 1988, pp. 266-279. As discussed in Soga, the concentrations of the internal and external donors in the catalyst system can be adjusted in order to optimize the activity and the stereospecificity of the catalyst. In the experimental work reported there, the transition metal catalyst component comprising titanium tetrachloride supported on magnesium dichloride with an internal donor, e.g., di-N butylphthalate, was slurried in hexane followed by the addition of an external electron donor, phenyl tri-ethoxysilane, and triethylaluminum (TEA) co-catalyst. Soga et al. report on polymerization rates over periods of several hours and isotactic indices measured over periods of several hours for various internal, external catalyst systems using several kinds of organosilicon compounds at varying concentrations expressed in terms of silicon titanium mole ratios and TEA/titanium mole ratios. Among the various electron donors used in the Soga et al. experimental work, diphenyldimethoxysilane appeared to have the most efficiency in terms of improving activity and/or stereospecificity of the catalyst system, followed by phenyltriethoxysilane, followed in turn by phenyltrimethoxysilane. Various other organosilicon compounds were generally less efficient, although still effective. The aluminum/titanium mole ratios employed in Soga range from about 50 to 200; the silicon/titanium mole ratios range from about 10 to 50.
U.S. Pat. No. 4,287,328 to Kikuta et al., is directed to the polymerization of alpha olefins in the presence of multi-component catalyst systems involving a xe2x80x9csolid productxe2x80x9d combined with an organoaluminum compound including, for example, C1-C10 trialkylaluminum, triethylaluminum, alkyl alkyoxyaluminums, and alkylaluminum halides, and an electron donor including various brganic acids, alcohols, ethers, aldehydes, ketones, amines, alkenol amines, esters, phosphines, phosphites, thioethers, thioalcohols, silanes, and siloxanes. The xe2x80x9csolid productxe2x80x9d catalyst component is formed by reacting a trivalent metal halide such as aluminum trichloride, aluminum tribromide or ferric trichloride with a divalent metal compound such as magnesium, calcium, or zinc hydroxide or oxide or carbonate with titanium tetrachloride, characterized as an electron acceptor. Numerous orders of additions of the various components are described in Kikuta et al., especially in columns 6 through 9. Conditions of mixing can vary over wide temperature ranges and time intervals, but temperatures are preferably in the range of room temperature to about 100xc2x0 C. The mixing of the various components can be carried out over periods of several minutes to several hours.
U.S. Pat. No. 4,567,155 to Tovrog et al., discloses multi-component catalyst systems useful in the gas phase polymerization of alpha olefins. In Tovrog et al., the catalyst systems comprise two base catalyst components, each containing subcomponents. The first component, identified as component xe2x80x9cAxe2x80x9d comprises a titanium component supported on a hydrocarbon insoluble magnesium component in combination with an electron compound. The second major component is a co-catalyst component, characterized as component xe2x80x9cBxe2x80x9d comprising a trialkylaluminum, an aromatic acid ester and an unhindered secondary amine. Tovrog discloses that the catalyst components may be mechanically activated by comminution prior to use in polymerization. Comminuted catalysts may be pre-polymerized with an alpha olefin before use as a polymerization catalyst component. In the pre-polymerization procedure, comminuted catalysts and an organoaluminum compound co-catalyst are contacted with an alpha olefin under polymerization conditions and preferably in the presence of a modifier such as methyl-p-toluate and an inert hydrocarbon such as hexane, with typical time durations for pre-polymerization and other pretreatment procedures involving periods of minutes up to a few hours.
U.S. Pat. No. 4,767,735 to Ewen et al. discloses a pre-polymerization process carried out over a period of less than a minute and usually ten seconds or less. In the Ewen et al. procedure, an organic solvent stream such as hexane or heptane is established in a pre-mixing line. To this stream are added sequentially a co-catalyst (TEAL), an external electron donor, diphenyldimethyoxysilane (DPMS) and a supported catalyst component (titanium tetrachloride supported on magnesium dichloride) to form a catalyst system which is then pre-polymerized by contact with propylene for a few seconds. An alternative mode of addition in the Ewen et al. procedure is to add the electron donor to the carrier stream after the addition of the titanium catalyst component, but still before the addition of the propylene. Ewen et al. disclose that the co-catalyst should be present when the electron donor and the transition metal catalyst component contact one another in order to avoid poisoning of the titanium catalyst. In the experimental work described in Ewen et al., one of two titanium chloride catalysts were used in conjunction with TEAL and DPMS under unspecified catalyst concentrations, but at relative amounts of TEAL and DPMS ranging from 2 mmol of TEAL and 0.4 mmol of DPMS (Al/Si ratio of 5) to 2 mmol of TEAL and 0.03 mmol of DPMS (Al/Si ratio of about 67). High efficiency catalyst systems employing external electron donors which may be characterized generally as sec or tert alkyl or cycloalkyl, alkyl dialkoxy silanes in combination with titanium tetrachloride supported on magnesium based supports derived from dialkoxy magnesium compounds are disclosed in U.S. Pat. No. 4,927,797 to Ewen. By way of example, the supported catalyst may be formulated through the reaction of diethoxy magnesium, titanium tetrachloride, and n-butyl phthalate under appropriate conditions as specified in the patent. A suitable external electron donor here is methylcyclohexyl dimethoxysilane, which is compared with diphenyldimethoxysilane as disclosed in the aforementioned Ewen et al. patent.
In accordance with the present invention there are provided novel processes for the formulation of Ziegler-type catalysts from catalyst components involving various orders of addition and mixing times between the several catalyst components. The catalyst components utilized in carrying out the invention involve a transition metal component, an organosilicon electron donor component, and an organoaluminum co-catalyst component which are sequentially mixed together in the course of formulating the Ziegler-type catalyst to be charged to an olefin polymerization reactor.
In carrying out the invention, different orders of addition of the several catalyst components can be used with the relative amounts of catalyst components varying depending upon the particular order of addition. In most cases, the preferred order of addition will involve initial mixing of the transition metal catalyst component with the organoaluminum co-catalyst component. These components are employed together to formulate a precursor mixture having a ratio of aluminum to transition metal mole (atomic) ratio of at least 200. The resulting precursor mixture is then combined with the organosilicon electron donor component to produce a Ziegler-type catalyst formulation in which the transition metal catalyst component and the electron donor component are present in relative amounts to provide an aluminum/silicon mole ratio of no more than 50. The mixture of the three components is then contacted with an olefin to effect polymerization of the olefin in the presence of the thus formulated Ziegler-type catalyst. Preferably, the olefin contacting step involves an initial pre-polymerization reaction to effect pre-polymerization of the catalyst prior to introducing the resulting pre-polymerized catalyst into a polymerization reactor containing an olefin. In this embodiment of the invention, it is preferred that the mole ratio of aluminum to silicon in the catalyst formulation be within the range of 20 to 50. The mole ratio of silicon to transition metal is at least 5 and more preferably, within the range of 10-20. Relatively short time sequences are preferably employed in formulating the Ziegler-type catalyst. More specifically, the precursor formulation of the organoaluminum co-catalyst and transition metal catalyst components is maintained for a time within the range of 5-120 seconds prior to subsequent contact with the organosilicon electron donor component. This then is followed by maintaining this formulation with the added electron donor component in contact for a period ranging up to 110 seconds prior to contact of the formulation with the olefin.
A further embodiment of the invention involves the formulation of a Ziegler-type catalyst as described above, by initial contact of the organoaluminum co-catalyst component with the organosilicon electron donor component. Here, the two components are used in relative amounts to provide a precursor mixture having an aluminum to silicon mole ratio of at least 10. This precursor formulation is then combined with the transition metal component to provide a formulation in which the aluminum/transition metal mole ratio is at least 200, followed by contact of the thus formulated catalyst mixture with an olefin to effect polymerization thereof in the presence of the catalyst mixture. The initial mixture of the organoaluminum and organosilicon compounds preferably is maintained for a time of 5-120 seconds and more preferably, in the range of 10-60 seconds prior to addition of the transition metal component. The resulting three component mixture is then maintained in contact for a period of up to 110 seconds, and preferably no more than 40 seconds prior to contact with the olefin in the subsequent polymerization step. Preferably, the silicon to transition metal mole ratio is at least 10; the aluminum to silicon mole ratios and the aluminum to transition metal mole ratios are within the range of 10-40 and 200-400, respectively.
In yet a further aspect of the invention, an order of addition is followed in which the organosilicon component and the transition metal catalyst component are initially mixed to form a precursor mixture having a mole ratio of silicon to transition metal of at least 5 for a first contact time up to 40 seconds and preferably no more than 30 seconds. This precursor mixture is then combined with the organoaluminum co-catalyst component to provide a Ziegler-type formulation having an aluminum/silicon ratio of no more than 40. The second contact time prior to contact of the Ziegler-type catalyst mixture with an olefin is such as to provide a total of the first and second contact times of no more than 60 seconds. In this embodiment of the invention, it is preferred that the silicon to transition metal mole ratio be within the range of 5-20.
As described previously, at the conclusion of the second contact time for various orders of addition, the resulting three component mixture is then contacted with an olefin to effect polymerization thereof in the presence of the catalyst mixture. For each of the various orders of addition it is preferred that a pre-polymerization step be carried out prior to introducing the catalyst into the main reactor. Preferably, the pre-polymerization is carried out for a relatively short period, usually of a duration of less than a minute and preferably less than 20 seconds.